Weight-loss, or bariatric, surgery helps you lose weight and lowers your risk of medical problems associated with obesity. Bariatric surgery contributes to weight loss in two main ways:

Restriction. Surgery is used to physically limit the amount of food the stomach can hold, which limits the number of calories you can eat.

Malabsorption. Surgery is used to shorten or bypass part of the small intestine, which reduces the amount of calories and nutrients the body absorbs.

Four common types of weight-loss surgery are:

Roux-en-Y gastric bypass

Laparoscopic adjustable gastric banding

Sleeve gastrectomy

Duodenal switch with biliopancreatic diversion

In Roux-en-Y gastric bypass, the surgeon creates a small pouch at the top of the stomach. The pouch is the only part of the stomach that receives food. This greatly limits the amount that you can comfortably eat and drink at one time.

The small intestine is then cut a short distance below the main stomach and connected to the new pouch. Food flows directly from the pouch into this part of the intestine. The main part of the stomach, however, continues to make digestive juices. The portion of the intestine still attached to the main stomach is reattached farther down. This allows the digestive juices to flow to the small intestine. Because food now bypasses a portion of the small intestine, fewer nutrients and calories are absorbed.

In the adjustable laparoscopic gastric banding procedure, a band containing an inflatable balloon is placed around the upper part of the stomach and fixed in place. This creates a small stomach pouch above the band with a very narrow opening to the rest of the stomach.

A port is then placed under the skin of the abdomen. A tube connects the port to the band. By injecting or removing fluid through the port, the balloon can be inflated or deflated to adjust the size of the band. Gastric banding restricts the amount of food that your stomach can hold, so you feel full sooner, but it doesn't reduce the absorption of calories and nutrients.

In a sleeve gastrectomy, part of the stomach is separated and removed from the body. The remaining section of the stomach is formed into a tube-like structure. This smaller stomach cannot hold as much food. It also produces less of the appetite-regulating hormone ghrelin, which may lessen your desire to eat. However, sleeve gastrectomy does not affect the absorption of calories and nutrients in the intestines.

As with sleeve gastrectomy, this procedure begins with the surgeon removing a large part of the stomach. The valve that releases food to the small intestine is left, along with the first part of the small intestine, called the duodenum.

The surgeon then closes off the middle section of the intestine and attaches the last part directly to the duodenum. This is the duodenal switch.

The separated section of the intestine isn't removed from the body. Instead, it's reattached to the end of the intestine, allowing bile and pancreatic digestive juices to flow into this part of the intestine. This is the biliopancreatic diversion.

As a result of these changes, food bypasses most of the small intestine, limiting the absorption of calories and nutrients. This, together with the smaller size of the stomach, leads to weight loss.

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